The present invention relates to a liquid metal ion source, a secondary ion mass spectrometer, and also a secondary ion mass spectrometric analysis method as well as the use thereof.
Secondary ion mass spectrometry is operated inter alia as so-called “Static Secondary Ion Mass Spectrometry” (SSIMS). An energy-rich primary ion beam is thereby directed onto a substrate surface to be analyzed. When impinging on the substrate, the primary ion beam strikes so-called secondary ions out of the material which are subsequently analyzed. From this analysis, the material of the surface can be determined. In order to obtain information about the distribution of specific substances over the surface, the primary ion beam can scan the surface. In order to obtain depth information, the beam is directed onto a specific place of the surface and removes the latter in the course of time so that also deeper layers can be exposed and analyzed. Also a combination of scanning of the surface with a depth profile analysis is possible.
A conventional static secondary ion mass spectrometric method is disclosed for example in DE 103 39 346 A1.
The method mentioned amongst experts under the name “Gentle Secondary Ion Mass Spectrometry” (Gentle SIMS or G-SIMS) has been available for some time for the analysis of surfaces. This is described for example in I. S. Gilmore et al. “Static SIMS: towards unfragmented mass spectra—the G-SIMS procedure”, Applied Surface Science 161 (2000) pp. 465-480.
This method termed G-SIMS was introduced in 1999 by I. S. Gilmore. The aim of the application of the G-SIMS method is to reduce the complexity of a TOF-SIMS spectrum (time-of-flight mass spectrometry) and to simplify the interpretation. This is because a TOF-SIMS spectrum has a large number of secondary ion lines/peaks. Such a secondary ion mass spectrometric spectrum is shown in FIG. 1. In addition to the characteristic lines for the polycarbonate sample examined here, this spectrum has a large number of intensive non-specific signals, such as for example polycyclic aromatic hydrocarbons. The conventional interpretation of such a spectrum presupposes empirical knowledge. Spectrum libraries are helpful in addition for the interpretation. Since the bombardment conditions have however great influence on the relative peak intensities of a TOP-SIMS spectrum, spectra of the same substance can deviate significantly from each other. With increasing numbers of primary ion sources and use of different bombardment conditions, the construction of a spectrum library is increasingly more difficult.
FIG. 2 now shows the same TOF-SIMS spectrum as in FIG. 1, but after application of the G-SIMS method. It is immediately obvious that the spectrum is very much simplified. Characteristic peaks are emphasised whilst non-specific fragments are suppressed. This significantly facilitates identification of molecular groups and the interpretation of the spectrum relative to the conventional TOF-SIMS spectrum. The application of the G-SIMS algorithm therefore makes it possible for the expert to have a rapid overview and delivers additional information for achieving a reliable interpretation of the data. Also easier access for interpretation of the data is made possible for the less experienced user.
Since G-SIMS spectra have only low dependency upon the bombardment conditions, the construction of spectrum libraries is substantially easier for the G-SIMS procedure than for the conventional TOF-SIMS method.
The G-SIMS method now presupposes the existence of two spectra with greatly different fragmentation behaviour. This fragmentation behaviour can be influenced very greatly by the energy and the mass of the primary ions which are used. In particular the influence of the primary ion mass on the fragmentation behaviour is important. This is because generally the fragmentation reduces with increasing mass of the primary ion. The strongest fragmentation can be achieved in contrast by choosing lighter high-energy atomic primary ions. By choosing heavier monoatomic or polyatomic bombardment particles, the emission spectrum is therefore displaced generally to higher masses and the fragmentation is significantly reduced. Uncharacteristic fragment peaks react therefore significantly more to the changed bombardment conditions than sample-specific molecular signals.
In the G-SIMS method which is mentioned in the above-mentioned publication by I. S. Gilmore et al. with more details than can be mentioned here, two spectra with greatly different fragmentation behaviour are recorded. After suitable normalisation, the spectra are divided such that signals which greatly differ in both spectra are suppressed. Signals which have only a small difference in both spectra are correspondingly amplified. A subsequent raising to a power of the quotient of both spectra increases these effects again significantly.
The G-SIMS method is in fact held in high regard, but in practical laboratory use, has to date only limited acceptance. This resides inter alia in the fact that the experimental complexity for this method is very great. This is because the required spectra with greatly different fragmentation behaviour can be achieved to date only by using different analysis sources.
On the one hand, gas ion sources are available which are able in principle to produce a series of differingly heavy atomic primary ions (Ar, Ne, Xe) and also polyatomic primary ions (SF5). The change between different species of primary ions is however very complex with these gas sources. Furthermore, these sources deliver only a restricted performance with respect to the achievable lateral resolution and mass resolution.
Alternatively, a plurality of different primary ion sources (e.g. with Ga or SF5) can be operated simultaneously. However, this demands high technical outlay, the achievable performance of such a G-SIMS analysis being limited by the weakest of the sources which are used. The required spectra can only be acquired here in succession so that the temporal complexity is very great.
Also commercially available cluster sources with Au or Bi have been proposed as emitter material. The emission spectra of such sources have both atomic and intensive polyatomic species. However is was quickly shown that the required strong variation in fragmentation with these sources could not be achieved. This is because the use of the clusters as analysis species leads in fact to spectra of low fragmentation, as are required for the successful application of the G-SIMS procedure. The maximum fragmentation in this case is however achieved by the use of the atomic species, the fragmentation being relatively low because of the large mass of the Au or Bi in the monoatomic primary ion beam and the achievable variation in fragmentation between the use of the clusters as primary ion beam and the use of monoatomic primary ions as primary ion beam not being sufficient.
To date, no primary ion sources which would be suitable for successful implementation of the G-SIMS method are therefore known.